Or without deleting the upper lip, you can simply hit CTRL while push/pulling to toggle the creation of a new starting face.

You will have some cleanup to do after as well as some face reverting or you could use ThomThom’s Solid Inspector 2 to help you with that. It’s a great tool to have if you plan on doing 3d Printing as objects needs to be considered solid for 3d printer to be able to print them.

Unless this was intentional, you might have an easier time starting with a clean rectangle, pulling it up to the proper bottom depth, then use Offset line to create the wall geometry, then pull them up.

Since you’re a newbie, it would be wise to set your enclosure aside for a little bit and work through the basic videos available on SketchUp.com so you can get the hang of using the tools. I think you’d find it time well spent.

It helps to pay attention to the status bar at the bottom, tool’s options are displayed and info is given as to what the tool expect you to do (selecting an edge when the tool is waiting for you to select a face can cause the tool to fail or give error messages for exemple)